Provide a configurable myshopify domain.

[garymardell]

@peterjm @christhomson @EiNSTeiN-

myshopify domain now becomes an option when setting up the omniauth builder. Defaults to myshopify.com.

I have also changed the valid_site? method to actually return a boolean in both cases and added a spec.

Rails.application.config.middleware.use OmniAuth::Builder do
  provider :shopify, ENV['SHOPIFY_API_KEY'], ENV['SHOPIFY_SHARED_SECRET'],
    myshopify_domain: 'myshopify.io'
end

I could rename the option, but it shouldn't be changed often.

[christhomson]

:mountain_cableway: :dash:

[peterjm]

:+1:

[EiNSTeiN-]

:+1:

[maartenvg]

Yes please! :+1: